Comet of the Century? New Comet ISON Views Herald an Amazing Show This Year

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An exceptional comet flying ever closer to the sun may offer an
amazing naked eye sight to Earth dwellers this fall as it
gradually brightens.

As comet
C/2012 S1 (ISON) continues to approach the sun, it is slowly
responding to the increasing warmth of the sun and getting
progressively brighter. The comet is getting considerable
scrutiny from both amateur and professional scientists because
it's a rare sungrazing comet, destined to approach to within
730,000 miles (1.17 million kilometers) of the surface of the sun
on Nov. 28. Because of this extremely close approach, comet ISON
holds the "potential" to flare into a dazzling object — possibly
becoming bright enough to be briefly glimpsed in broad
daylight.

Although still quite far from the sun and very faint, the comet
has been imaged by two orbiting observatories. Astronomers from
the University of Maryland at College Park and Lowell Observatory
used NASA's
Swift satellite to check out the comet during January and
February. Using images acquired from Swift's Ultraviolet/Optical
Telescope, the team was able to make initial estimates of the
comet's water and dust production and then used these values to
determine the size of ISON's icy nucleus.

These observations revealed that each minute ISON was shedding
about 56 tons (51,000 kg) of dust, or about two-thirds the mass
of an unfueled space shuttle. Jets powered by sublimating ice
also release dust, which reflects sunlight and brightens the
comet.

By contrast, however, the
comet was producing only about 130 pounds (60 kg) of water
every minute, or about four times the amount flowing out of a
residential sprinkler system. At the time, however, the comet was
nearly half a billion miles from the sun. Typically, a comet's
water content remains frozen until it comes within about three
times Earth's distance to the sun — about 280 million miles (450
million km) away. ISON won't be this close to the sun until early
July at which time the water production rate should markedly
increase.

The water and dust production rates from Swift were used to
estimate the size of ISON's icy body. Comparing the amount of gas
needed for a normal comet to blow off dust at the rate observed
for ISON, the scientists estimate that the nucleus is roughly 3
miles (5 km) across, a typical size for a comet. This assumes
that only the fraction of the surface most directly exposed to
the sun, about 10 percent of the total, is actively producing
jets.

Hubble's turn

More recently, Planetary Science Institute research scientist
Jian-Yang Li led a team that imaged
comet ISON with the Hubble Space Telescope on April 10 using
the Wide Field Camera 3. At this point, the comet was slightly
closer than Jupiter at 386 million miles (621 million kilometers)
from the sun and 394 million miles (634 million kilometers) from
Earth.

The comet's dusty coma, or head of the comet, is currently about
3,100 miles (5,000 kilometers) across, or 1.2 times the width of
Australia. A dust tail extends more than 57,000 miles (92,000
kilometers), far beyond Hubble's field of view.

A detailed analysis of the dust coma surrounding the nucleus
reveals a strong jet blasting dust particles off the sun-facing
side of the comet's nucleus. This jet, as projected on the sky,
extends at least 2,300 miles (3,700 kilometers).

A "promising" future

What does all of this mean regarding the comet's performance
later this year?

"It looks promising, but that's all we can say for sure now,"
said Matthew Knight, an astronomer at Lowell Observatory in
Flagstaff, Ariz., and a member of the Swift and Comet
ISON Observing Campaign teams. "Past comets have failed to
live up to expectations once they reached the inner solar system,
and only observations over the next few months will improve our
knowledge of how ISON will perform."

We'll just have to wait and see how ISON evolves in the coming
weeks and months. Stay tuned to SPACE.com for future
updates.

Editor's note: If you capture an amazing
photo of Comet ISON or any other night sky sight that you'd like
to share for a possible story or image gallery, please contact
managing editor Tariq Malik at spacephotos@space.com.

Joe Rao serves as an instructor and guest lecturer at New
York's Hayden Planetarium. He writes about astronomy for
The New York Times and other publications, and he is also an
on-camera meteorologist for News 12 Westchester, New
York. Follow us @Spacedotcom, Facebook and Google+.
Original article on SPACE.com.